In deep well drilling it is common practice to take measurements continuously while drilling by means of measurement systems installed in the drill string and to transmit the measurement results to the surface of the earth by means of telemetry devices. To generate the electric power required to operate the measurement systems and telemetry devices, use is generally made of a generator which is driven by a turbine arranged in the drill string. The turbine draws its drive energy from the flow of drilling fluid which is fed through the drill string to the drill bit. The problem encountered with this approach however is that the feed rate of the drilling fluid fed through the drill string is dependent on the drilling conditions such as pump capacity, well depth and physical properties of the drilling fluid, to name but a few, and can be subject to severe fluctuations on a scale of 1 to 4. Such fluctuations are unsuitable for the drive of the turbine and the generator connected thereto and would lead to hardly controllable fluctuations of rotational frequency and performance. It is necessary therefore to limit the feed rate of drilling fluid acting on the turbine rotor and to supply to the turbine, independently of the feed rate of the mud pump, only that feed rate required to achieve the desired drive performance.
From EP 0 069 530 A2 is known a bypass device for a turbine which is arranged in a drill string and has a valve which is arranged upstream from the turbine in the drill string in order to control the flow of fluid directed past the turbine. The valve is actuated by a piston arrangement which is acted upon in one direction by the pressure on the output side of the turbine and a compression spring, and in the opposite direction by the pressure on the input side of the turbine. The position of the valve varies in response to the pressure differential between input and output, thereby regulating the quantity of drilling fluid which gets to the turbine input and the quantity which flows past the turbine. By this means the output performance of the turbine should be maintained essentially constant in spite of changing operating conditions.
The known bypass device has the disadvantage that the pressure differential between the input and output of the turbine is dependent on the volumetric flow fed to the valve and the turbine on the input side and increases in the same proportion as the volumetric flow. Hence the device operates in the manner of a volumetric divider in which an increasing volumetric flow at the input produces not only an increase in the bypass flow but also an increase in the flow passing through the turbine. Severe fluctuations of the input flow thus lead also to severe fluctuations of the turbine flow and hence also of the turbine output performance, particularly since said performance increases as a rule more than proportional with the turbine current. The known device is therefore unsuitable for decoupling the turbine performance sufficiently from the fluctuations of the drilling fluid supply.
In addition there is known from JP 04022766 A a speed controlling device for a turbine generator arranged in a drill string, wherein a valve is arranged at the turbine input and held in an open position by spring force. A bypass channel bypassing the turbine is provided parallel to the input of the valve. In this arrangement, the valve is increasingly closed as the feed rate of the supplied drilling fluid increases so that the bypass rate increases while the volumetric flow which reaches the turbine is kept essentially constant. This device has the disadvantage that a relatively large bypass cross section is always open so that in the presence of small feed rates the flow flowing to the turbine is too small. In addition there is the risk, particularly with the valve closed to greater degrees, of the valve passage becoming clogged with dirt particles entrained in the drilling fluid so that the drive performance of the turbine drops too severely or even that the turbine stops and the power supply collapses as the result.